1. Field of the Invention
The present invention relates to a method of processing original information of a still image or moving image, more particularly relates to a method of coding that original image information and decoding that coded information. Further, the present invention relates to an encoder and a decoder for carrying out that method.
2. Description of the Related Art
When coding original image information and transmitting it to a reception side, in order to improve the coding efficiency of coding a pixel block to be coded, use is made of the so-called predictive coding method for coding a target pixel block by predicting the target pixel block from peripheral pixel blocks located in its vicinity. A variety of these predictive coding methods have been established as standard methods in the past. As one of these, the ITU-T standard H.264 has been proposed in recent years. If the present invention is applied to for example an image coding method based on this H.264, it would become possible to further raise the coding efficiency, that is, the data compression efficiency.
Note that as known art concerning the present invention, there are Japanese Patent Publication (A) No. 8-317401 and Japanese Patent Publication (A) No. 2002-77635. Japanese Patent Publication (A) No. 8-317401 discloses, when a binary image is divided into several frames, to analyze these divided frames so as to determine the coding method of the nearby frames and thereby improve the coding efficiency. Further, Japanese Patent Publication (A) No. 2002-77635 discloses, when coding a multi-value image, to adaptively change the pixel which is referred to in order to obtain a predictive value of DPCM in accordance with the statuses of the peripheral pixels. However, neither include the concept of the basis of the present invention, that is, “control of the coding order”.
FIGS. 14A and 14B are diagrams showing a general coding order (FIG. 14A) and an optimal coding order (FIG. 14B). FIGS. 14A and 14B show portions in the original image where for example a first color (blanks) and a second color (hatchings) obliquely contact. Further, in the figures, the block boundaries (BB) show the boundaries of the pixel blocks now being coded. These pixel blocks are divided into for example four sub-blocks SB.
When sequentially processing these four sub-blocks SB, the coding order thereof, considering the raster scanning of TV signals, generally becomes as indicated by the white arrows Co (coding order) in the figures. Accordingly, the coding order of sub-blocks SB in FIG. 14A becomes “0→1→2→3”. However, this raster scanning involves the following problem.
First, in the case of a pattern of the image shown in FIG. 14A, the direction of prediction from the peripheral sub-blocks becomes as indicated by the arrows PR (prediction) in the figure. However, FIG. 14A shows an example of the extrapolative prediction direction (PR) among general interpolative prediction and extrapolative prediction. This being the case, the following inconvenience arises in a portion sb indicated by the double hatching in FIG. 14A.
When coding this portion sb, the sub-block SB which should be referred to becomes the sub-block SB (3) just beneath that sb, according to the above-described prediction direction. However, this sub-block SB (3) has not yet finished being coded judging from the above coding order CO. That is, such a not yet coded sub-block SB (3) cannot be referred to. This being the case, the problem arises that the related portion sb ends up remaining as a portion unable to be predictively coded.
Therefore, if trying to solve this problem, the optimal coding order such as shown in FIG. 14B may be employed. According to the coding order of FIG. 14B, the problem of the portion sb unable to be predictively coded shown in FIG. 14A is solved. This is because the sub-block SB (2) just beneath of that portion sb finishes being coded earlier.
However, when employing the optimal coding order of FIG. 14B, another problem arises. This is the problem that it is usually difficult to determine the above optimal coding order adaptively and in a short time for each pixel block (block surrounded by BB), so the prediction efficiency of predicting the pixel block drops and as a result the coding efficiency drops.
If trying to determine the optimal coding order of FIG. 14B adaptively and in a short time, it becomes necessary to add additional information for designating the optimal coding order for each pixel block. However, if trying to do this, then another problem is caused of the coding efficiency dropping by the addition of the above additional information.